In treatment of domestic and industrial wastewater, aeration is one of the processes commonly used to promote biological consumption and removal of dissolved and suspended waste material. Aeration devices, called diffusers, are mounted at submerged locations in a man-made or natural wastewater impound, such as a tank or lagoon. Air and/or other treatment gas, in most instances composed of or containing some form of oxygen, is supplied to the diffusers in bulk and is discharged from them as multitudes of tiny bubbles. As these bubbles rise buoyantly through the wastewater, oxygen in the bubbles dissolves into the wastewater. Oxygen supports the life processes of bacteria, supplied to the wastewater in the treatment process, and these bacteria consume the waste. Other treatment gases (including vapors), and sometimes liquids, not necessarily containing oxygen, may be passed through the diffusers for a variety of purposes, such as for cleaning them.
Payments for electricity consumed by compressors or blowers that supply air and/or other treatment gas to the diffusers is one of the largest costs, if not the largest cost, of operating a wastewater treatment plant. Accordingly, much effort has been expended, by those working in this art, to enhance the efficiency of diffuser systems, including not only the diffusers themselves but also arrangements of and ways of operating diffusers within the plants. Moreover, efforts have been made to simplify, “ruggedize” and therefore reduce the capital and maintenance costs of diffuser systems. These efforts have led to a stream of improvements in wastewater treatment plant and diffuser design.
One popular type of diffuser that has been the focus of continuing research and development effort is the membrane diffuser. A membrane diffuser generates tiny gas bubbles by passing treatment gas into wastewater under pressure through a myriad of minuscule pores extending through relatively thin but tough rubbery material in the form of, for example, tubes, rectangular sheets, or disks that are of circular outline in plan view. These pored rubbery media, dubbed membranes, are typically secured in gas-tight relationship, e.g. by a clamping arrangement, to a suitable holder, referred to as a diffuser body.
FIGS. 1-5 depict one particularly popular type of membrane diffuser system which has been available from Sanitaire Division of ITT Industries and its predecessors for more than a decade. FIGS. 1-2 show that in such systems there is a diffuser 1 which includes a body 2 having a saddle-shaped lower wall 3 secured to the upper surface of a gas supply conduit 4 of circular cross-section. The body also includes an inclined conical wall 5, the upper, inner edge of which includes a shelf 6, upon which rests support plate 7. Surrounding plate 7 is a vertical sidewall 10 of the body, an upward extension of shelf 6 having threads 11 on its outer surface. A threaded ring 12 having inwardly projecting flange 13 is installed on threads 11. Membrane 14 includes a central portion 15 and, at its periphery, an integral O-ring portion 16 which is held in sealing engagement with the underside of flange 13, the inner surface of sidewall 10 and a step 17 formed in a peripheral upper edge of plate 7.
In the operation of such a diffuser, treating gas flows from the interior 20 of gas supply conduit 4 through an orifice 21 in the crown of the conduit, acting as a flow regulator. The treating gas enters the diffuser through gas inlet port 22 in lower wall 3 of the body and then passes through a plenum 23 within the body, through a gas passage 24 in support plate 7, through a gas chamber 25 formed between the upper surface of support plate 7 and the lower, gas influent surface of membrane 14, which is inflated when gas is flowing through the diffuser, and finally through perforations 26 in the membrane.
FIGS. 3-5 to illustrate the installation of such diffusers in a wastewater treatment plant. These figures portray schematically a wastewater treatment tank 30 having sides 31 (only one of which is shown), ends 32 and bottom 33. With the aid of conventional stands (not shown) secured to tank bottom 33, a number of the previously mentioned gas supply conduits 4 are mounted close to the bottom in a parallel array. Large numbers of diffusers 1 are mounted at spaced intervals along the gas supply conduits 4, and those conduits are connected through manifold 34 and downcomer pipe 35 to a source of treatment gas under pressure, such as one or more blowers or compressors (not shown).
In many wastewater treatment plants, the wastewater passes through a series of tanks, for example as illustrated in FIG. 5. The density of diffusers, that is, the number of diffusers, and thus the amount of diffuser discharge area per unit of tank bottom area, can be varied from tank to tank or within a given tank, depending on the requirements of the wastewater and of the particular type of treatment being performed. In certain instances, portions of the tank may have no diffusers installed, thereby facilitating, for example, in an aeration plant, the creation of anoxic zones. As persons skilled in the art will readily understand, there are many different ways of laying out the diffusers and gas supply conduits in wastewater treatment plants, and the subject matter depicted in these figures represents merely a sample rather than a comprehensive illustration of prior practice.
The diffusers illustrated in FIGS. 1-5, when viewed in plan view, are of circular outline, and are thus referred to as membrane disk diffusers. The particular diffusers illustrated above provides very high performance in terms of system durability and OTE (oxygen transfer efficiency) and, as such, have achieved wide acceptance in many countries throughout the world.
Numerous other membrane diffuser designs have developed, including membrane tube diffusers, based on tubular membranes, and panel diffusers, based for example on rectangular sheets of membrane material. Typically, they permit a modest degree of inflation of portions of the membrane surfaces by the pressurized treatment gas. Because of the clamping of disk and sheet membrane edges, inflation occurs inward of those edges. Membrane disk diffusers may or may not be restrained against inflation at their centers. With the rectangular sheet membranes of panel diffusers, the area of the membrane and the resultant potential for inflation are often considerably larger than in the disks. Thus, some type of overlying grid member with relatively large openings in it is usually included in the diffuser body and held against the upper surface of the membrane to control the extent to which it inflates.
Whether as a result of unexpected power failures or intentional shut-off for process reasons, membrane diffusers can undergo interruption of gas flow, resulting in deflation of the membranes. In view of their submergence and the great weight of the wastewater bearing down on the membranes, membrane diffuser bodies ordinarily include some sort of membrane support beneath the membrane to prevent it from being damaged or displaced under the weight of the wastewater when the membrane is no longer supported by gas pressure. When gas is flowing and the membrane is under pressure and at least partially inflated, a space or gas chamber exists between the upper surface of the support and the lower surface of the membrane.
Another category of membrane diffuser that has evolved is the strip diffuser. For example see U.S. Pat. Nos. 4,029,581 and 5,868,971; U.S. Published Patent Application US2002/0003314 A1; International (PCT) Published Application WO 98/21151; and Offenlegungschrift (German Published Application) DE 42 40 300 A1. The term strip is appropriate for these diffusers because their membranes and gas discharge surfaces generally have a length to width ratio larger than that found in the typical panel diffuser. For example, length to width ratios of about 4:1 or more, and in some cases considerably larger, can be found in strip diffusers.
Where there is this greater length to width ratio, it is possible to provide the diffuser with considerable aeration area while limiting its width. Diffuser area, utilized properly, can be a factor in attaining desired or increased levels of OTE (oxygen transfer efficiency), with resultant conservation of electricity during processing of a given amount of wastewater. Strip diffusers hold promise of a convenient way of increasing the mass transfer rate of oxygen into wastewater while maintaining OTE levels at least approximately consistent with disk diffusers. Also, in many instances it is possible to limit the width of the membrane in a strip diffuser to a sufficient extent that an overlying grid member and its attendant manufacturing costs can be dispensed with. On the other hand, in common with panel diffusers, strip diffusers include membranes and diffuser bodies which include membrane supports.
Strip diffusers are believed to represent a promising approach for further reducing the capital costs, including those of installation, and the operating costs, of biological wastewater treatment plants involving aeration. It is believed that there is room, and a need for, further improvements in strip diffusers, and the subject matter of the present disclosure and claims is aimed at fulfilling this need.